Method for centering a sputter target onto a backing plate and the assembly thereof

ABSTRACT

A method for aligning the sputter target onto a backing plate having a peripheral arcuate-shaped flange on its bonding surface to provide an aligned and uniform solder bonded interface; and the sputter target/backing plate assembly so produced.

FIELD OF THE INVENTION

The invention relates to a method of bonding a sputter target to abacking plate, and more specifically, the use of a backing plate havingat least two spaced-apart peripheral flange segments on the bondingsurface of the backing plate so that the flange segments provide acentering for the sputter target and also provide a uniform spacingbetween the target and backing plate to accommodate a uniform solderbonded interface.

BACKGROUND OF THE INVENTION

Cathodic sputtering is widely used for the deposition of thin layers ofmaterial onto desired substrates. Basically, this process requires a gasion bombardment of a target having a face formed of a desired materialthat is to be deposited as a thin film or layer on a substrate. Ionbombardment of the target not only causes atoms or molecules of thetarget materials to be sputtered, but imparts considerable thermalenergy to the target. This heat is dissipated beneath or around abacking plate that is positioned in a heat exchange relationship withthe target. The target forms a part of a cathode assembly that, togetherwith an anode, is placed in an evacuated chamber filled with an inertgas, preferably argon. A high voltage electrical field is applied acrossthe cathode and the anode. The inert gas is ionized by collision withelectrons ejected from the cathode. Positively charged gas ions areattracted to the cathode and, upon impingement with the target surface,these ions dislodge the target material. The dislodged target materialtraverses the evacuated enclosure and deposits as a thin film on thedesired substrate, which is normally located close to the anode.

In addition to the use of an electrical field, increasing sputteringrates have been achieved by the concurrent use of an arch-shapedmagnetic field that is superimposed over the electrical field and formedin a closed loop configuration over the surface of the target. Thesemethods are known as magnetron sputtering methods. The arch-shapedmagnetic field traps electrons in an annular region adjacent to thetarget surface, thereby increasing the number of electron-gas atomcollisions in the area to produce an increase in the number of positivegas ions in the regions that strike the target to dislodge the targetmaterial. Accordingly, the target material becomes eroded in a generallyannular section of the target face, known as the target raceway.

In a conventional target cathode assembly, the target is attached at asingle bonding surface to a nonmagnetic backing plate to form a parallelinterface in the assembly. The backing plate is used to provide a meansfor holding the target assembly in the sputtering chamber and to providestructural stability to the target assembly. Also, the backing plate isnormally water-cooled to carry away the heat generated by the ionbombardment of the target. Magnets are typically arranged beneath thebacking plate in well-defined positions to form the above-noted magneticfield in the form of a loop or tunnel extending around the exposed faceof the target.

To achieve good thermal and electrical contact between the target andthe backing plate, these members are commonly attached to each other byuse of soldering, brazing, diffusion bonding, mechanical fastening orepoxy bonding.

Smooth surface diffusion bonding is an applicable method of bonding, buthas only limited use in the bonding of sputtering target components. Thebond is produced by pressing the material surfaces into intimate contactwhile applying heat, to induce metallurgical joining and diffusion to avarying extent across the bond interface. Bonding aids, metalcombinations which are more readily joined, are sometimes applied to oneor both of the surfaces to be bonded. Such coatings may be applied byelectroplating, electroless plating, sputtering, vapor deposition orother usable techniques for depositing an adherent metallic film. It isalso possible to incorporate a metallic foil between bonding membersthat has the ability to be more easily bonded to either of the materialsto be joined. The surfaces to be joined are prepared by chemical orother means to remove oxides or their chemical films which interferewith bonding.

Smooth surface diffusion bonding requires extreme care in preparationand in maintaining surface cleanliness prior to and during the bondingoperation to ensure reliable bond qualities. Because the diffusion bondinterfaces are planar, they are subject to stressing in simple shearwhich commonly leads to peeling away at the ends of the bond area. Theformation of brittle intermetallics at the bond interface, whichincrease in thickness with the associated long times of heat exposure,add to the potential of bond shear failure. An additional technique forbonding, as described in U.S. Pat. No. 5,230,459 includes thepre-bonding step of providing machined grooves in the surface of one ofthe components to be solid-state bonded. This feature causes disruptionof the bond surface of the associated component during heated pressureapplication. The material having the greater strength of hardness willnormally be provided with the grooves such that, during bonding, it willpenetrate into the softer member with the softer metal substantiallyfilling the grooves.

Groove bonding is applicable to bonding many dissimilar materials, butis limited to materials that have dissimilar melting temperaturesbecause the process must occur near the melting temperature of the lowermelting point alloy. This precludes the use of this technique forsimilar metals. It is also possible that the saw tooth nature of thegrooves may act as a stress concentrator and promote premature crackingin the alloys near the bonds. Furthermore, machining of the grooves is atime consuming operation.

In U.S. Pat. No. 5,836,506, hereby incorporated by reference in itsentirety, a method is disclosed for performing a surface rougheningtreatment to the bonding surface of the sputter target and/or backingplate, followed by solid state bonding. This roughening surfacetreatment provides 100% surface bonding compared to only 99% surfacebonding in the absence of the surface treatment. The treatment furtherprovides a bond with over twice the tensile strength of a bond formedfrom the non-treated smooth surfaces.

In all of the above diffusion bonding processes, elevated temperaturesof varying degree are applied to form the bond between the target andthe backing plate. Thus, in each of these processes, changes in themicrostructures of the target and backing plate materials are likely tooccur because prolonged exposure of metals to elevated temperaturescauses grain growth. Great strides have been made in this art to processsputter target blanks to achieve certain microstructures that are linkedto increased sputtering efficiency and improved thin film quality. Aftera desired microstructure is obtained in the sputter target, themicrostructures could be in jeopardy of being altered by elevatedtemperature bonding methods for attaching the target to the backingplate.

Additionally, although diffusion bonding has been proven successful,extra large target/backing plate assemblies require large scalediffusion bonding presses and this poses a significant capitalexpenditure.

Prior art attempts to solve the problem of distorting the microstructureof the sputter target and maintaining a consistent and uniform bond,requires the placing of wire gauges between the sputter target andbacking plate. This method is labor intensive, costly and the wirespacer gauges tend to move during the bonding process. Additionally,when the wire gauges are used on the outer edges of the sputter targetassembly, bowing of thin or large targets can occur due to aninconsistent thickness of sputter target material. Thickness uniformityof sputter targets is particularly important for ferromagnetic materialsin order to achieve good thickness and sheet resistance uniformity ofsputtered films.

It is an object of the invention to provide a method of forming a bondedsputter target/backing plate assembly that has a uniform thickness bondinterface and uniform flatness of the target sputtering surface.

Another object of the invention is to provide a bonded sputtertarget/backing plate assembly that does not compromise themicrostructural characteristics of the sputter target.

Another object of the invention is to provide a bonded sputtertarget/backing plate assembly having at least two spaced-apartperipheral flange segments disposed on the bonding surface of thebacking plate and at least two peripheral notch segments to accommodatethe at least two peripheral flange segments on the backing plates.

SUMMARY OF THE INVENTION

In one embodiment of the invention, there is provided a method forforming a solder bonded sputter target/backing plate assembly comprisingthe steps of:

-   -   a) forming a backing plate with a bonding surface having at        least two spaced-apart peripheral flange segments disposed on        the bonding surface of the backing plate;    -   b) forming a sputter target having a sputter surface and at        least two peripheral notched segments on the bonding surface and        said notched segments adapted for aligning with the flange        segments;    -   c) applying a solder material to the interface spacing defined        by superimposing and aligning said sputter target on the backing        plate and said flange segments having a height larger than the        depth of the notched segments; and    -   d) allowing said solder material to solidify and bond the        sputter target to the backing plate.

Another embodiment of this invention there is provided a method forforming a solder bonded sputter target/backing plate assembly comprisingthe steps of:

-   -   a) forming a backing plate with a bonding surface having at        least two spaced-apart peripheral flange segments and a spacer        element disposed on the inner walls of the flange segments and        the peripheral edge surface of the backing plate, and said        spacer elements having a height equivalent to the height of the        intended solder bonded interface;    -   b) forming a sputter target having a sputter surface and a        bonding surface and having a size to accommodate the        spaced-apart flange segments;    -   c) applying a solder material to the interface spacing defined        by superimposing and aligning said sputter target between the        flange segments and onto the spacer elements;    -   d) allowing said solder material to solidify and bond the        sputter target to the backing plate; and    -   e) removing said flange segments from the backing plate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the preferred embodiment of the invention, the solder bonded sputtertarget/backing plate assembly would be disc-shaped and have at least twoarcuate-shaped flanges spaced apart on the maximum degree separation ofthe bonded surface of the disc-shaped backing plate. Preferably, thebacking plate and flange segments are made of the same material and theheight of the flange segments should be constant so that a uniformthickness solder bonded interface can be achieved along with providing acentering means for the sputter target. For most applications, theheight of the flange segments could vary between about 0.100 inch andabout 0.500 inch, preferably between about 0.100 inch and about 0.300inch and most preferably about 0.200 inch. Preferably, the thickness ofthe flange is between about 0.200 inch and about 0.300 inch.

The spacing of the flange segments has to be sufficient to permitcentering of the sputter target, especially for thinner and largediameter sputter targets. Preferably, the flange segments form a singlearcuate-shaped flange, i.e., ring, and have a uniform height asdiscussed above. In the preferred embodiment, the notch segments form asingle arcuate-shaped peripheral notch and could have a depth smallerthan the height of the arcuate-shaped flange of a mating backing plate.Preferably the depth of the arcuate-shaped notch should be between about5% and about 20% of the height of the flange, more preferably betweenabout 10% and about 15%. This difference between the notch's depth andthe flange's height is the thickness of the solder bonded interface ofthe sputter target and the backing plate. Consequently the thickness ofthe solder bonded interface will vary depending on the size of thesputter target used. The thickness of the width of the flange segmentscould vary, depending on the application, and preferably be betweenabout 0.100 inch and about 0.500 inch, more preferably between about0.100 inch and about 0.400 inch and most preferably about 0.250 inch.Again, the width of the notch can be between 10% and about 50% of thewidth of the flange, preferably between about 20% and about 40% and mostpreferably between about 20% and about 30%.

In the embodiment using a spacer element, the dimensions of the flangesegments would be as discussed above and the spacer element couldpreferably be an annular ring-shaped having a height of between about0.010 and about 0.030 inch, and preferably between about 0.015 and about0.025 inch. This height dimension is the thickness of the solder bondedinterface of the sputter target and backing plate. Preferably the radialwidth of the spacer element could be between about 0.010 inch and about0.050 inch, more preferably between about 0.015 inch and about 0.030inch and most preferably about 0.020 inch. The flange could be aremovable flange so that the backing plate could be reused for otherapplications.

The metals used for the sputter target and backing plate may be any of anumber of different metals, either in pure or alloy form. For example,the sputter target may be made of titanium, aluminum, copper,molybdenum, cobalt, chromium, ruthenium, rhodium, palladium, silver,osmium, iridium, platinum, gold, tungsten, silicon, tantalum, vanadium,nickel, iron, manganese, germanium, or alloys thereof. The backing platecould be made of copper, aluminum, titanium, or alloys thereof.Preferred sputter target/backing plate metal pairings include a titaniumtarget bonded to an aluminum backing plate; a titanium target bonded toa copper backing plate; a titanium target bonded to a titanium backingplate; a molybdenum target bonded to a copper backing plate; a cobalttarget bonded to a copper backing plate; a chromium target bonded tocopper backing plate; and a target formed of a precious metal such asruthenium, rhodium, palladium, silver, osmium, iridium, platinum orgold, bonded to a copper backing plate. If a titanium-tungsten alloy isused, the alloy preferably includes about 10% to 15% titanium by weight.

Although the method has been described in conjunction with a disc-shapedsputter target/backing plate assembly, it will be readily apparent toone of ordinary skill that the method may be used to bond sputtertargets and backing plates having any of a number of different shapesand sizes.

On large size sputter targets, the flange segment may not be sufficientto prevent bowing or other distortion of the sputter target and thus atleast one protruding ridge can be disposed within the flange andcentered to provide a support for the sputter target. The height of theprotruding ridge is preferably the height of the flange. The shape ofthe protruding ridges could be arcuate, circle, square, rectangular,polygon and a combination thereof.

Suitable solder materials would be liquid or paste solders comprising alow melting metal containing component or a mechanically alloyed solidsolution component. Examples of suitable solder materials are tin-lead,indium-tin, tin-silver, tin-copper, or tin-silver-copper. Preferably,the flange segments could provide at about one opening so that thesolder could flow easily between the sputter target and backing plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross and sectional view of a sputter target/backing plateassembly showing the sputter target separate from the backing plate.

FIG. 2 is a cross-sectional view of another sputter target/backing plateassembly showing the sputter target separate from the backing plate.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a sputter target/backing plate assembly 2 is shownhaving a sputter target 4 and a backing plate 6. The backing plate 6 ismade with arcuate-shaped flange 8 to provide a centering means to centerthe sputter target 4 upon the backing plate 6.

The height A of flange 8 is larger than the depth B of the notch 10 asdiscussed below. When the sputter target 4 is superimposed on thebacking plate 6 via flange 8 there is an interface spacing of A-B. Thisspacing (not shown) is used to accommodate a solder material to securethe sputter target 4 to the backing plate 6. Although not shown, theflange could have two segments spaced 180° apart, three segments spaced120° apart, four segments spaced 90° apart, etc. When using more thanone flange arrangement, a spacing is available to permit a soldermaterial to flow into the spacing (A-B) formed at the interface of thesputter target 4 at the backing plate 6. As shown in FIG. 1, the widththickness C of the backing plate 6 is the same as the radial edge D ofthe sputter target 4. Solder will flow in the space A-B and also in theinterface of B-D and A-C to provide a stronger bond between the sputtertarget 4 and the backing plate 6. In some applications, such s largesputter targets, one or more protruding ridges 12 could be disposed onthe bonding surface 14 to provide support to the sputter target 4.

FIG. 2 shows a sputter target/backing plate assembly 20 having adisc-shaped sputter target 22 and a disc-shaped backing plate 24. Thebacking plate 24 has a peripheral flange 26 to provide a centering meansto center sputter target 22 onto the backing plate 24. Preferably heightE is smaller than height F so that weight or pressure may be applied totarget face (34).

The slanted flange leg 28 is shown with a dotted line 30 indicatingwhere the area 32 can be removed by any conventional means. The removalof area 32 will provide the sputter target with an unobstructedsputtering surface 34. In another embodiment of this assembly 20, thearea 32 defined by line 30 would be secured to the backing plate 24 witha removable means, such as screw 36. This would permit the backing plateto be reused for multiple applications. Spacer 38, such as an annularring, could be used to provide an interface space between the sputtertarget 22 and backing plate 24 to accommodate a solder material (notshown). The height of the spacer 38 will be the thickness of the solderbonded interface upon assembly of the sputter target and backing plate.

The method of this invention is especially useful for solder bonding 30mm magnetic alloy sputter targets as the nature of the material requiresthe thinner target configuration, with a greater sputter diameter, andtherefore increasingly difficult to obtain a uniform bond layerthickness.

Advantages of this invention are consistent centering of the sputtertarget upon the backing plate, consistent solder bonded layer thickness,reduction in preparation time, uniform flatness of the sputter targetsurface, and the ability to provide larger sputter target diameters witha uniform thickness solder bonded interface.

While the present invention has been illustrated by the description ofan embodiment thereof, and while the embodiment has been described inconsiderable detail, it is not intended to restrict or in any way limitthe scope of the appended claims to such detail. Additional advantagesand modifications will readily appear to those skilled in the art. Theinvention in its broader aspects is therefore not limited to thespecific details, representative assembly and method shown anddescribed. Accordingly, departures may be made from such details withoutdeparting from the scope or spirit of applicants' general inventiveconcept.

1. A method for forming a solder bonded sputter target/backing plateassembly comprising the steps of: a) forming a continuous solid backingplate with a bonding surface having at least two spaced-apart peripheralflanged segments disposed on the bonding surface of the backing plate;b) forming a sputter target having a sputter surface and a bondingsurface and at least two peripheral notched segments on the bondingsurface, and said peripheral notched segments adapted for aligning withthe peripheral flange segments; c) applying a solder material to aninterface spacing defined by superimposing and aligning said bondingsurface of the sputter target on the bonding surface of the backingplate and said peripheral flange segments having a height thicknesslarger than the depth thickness of the peripheral notched segments; andd) allowing said solder material to solidify and bond the sputter targetto the backing plate.
 2. The method of claim 1 wherein the backing plateand sputter target are disc-shaped.
 3. The method of claim 1 wherein theflange segments form a single arcuate-shaped flange and the notchedsegments form a single arcuate-shaped notch.
 4. The method of claim 3wherein the height of the flange is between about 0.100 inch and 0.500inch.
 5. The method of claim 3 where the thickness of the width of theflange is between about 0.100 inch and about 0.500 inch.
 6. The methodof claim 1 wherein the depth of the notch is between about 0.010 inchand about 0.030 inch.
 7. The method of claim 3 wherein the difference inthe height thickness of the flange is between about 5% and about 20%larger than the depth thickness of the notch.
 8. The method of claim 3forming the backing plate with at least one protruding ridge within theflange and on the bonding surface of the backing plate and said ridgehaving a width of between about 0.100 inch and about 0.500 inch.
 9. Themethod of claim 8 wherein the at least one ridge on the bonding surfaceof the backing plate have a shape selected from the group comprising acircle, arcuate, square, rectangular, polygon and combination thereof.10. The method of claim 1 wherein the sputter target is selected fromthe group comprising titanium, aluminum, copper, molybdenum, cobalt,chromium, ruthenium, rhodium, palladium, silver, osmium, iridium,platinum, gold, tungsten, silicon, tantalum, vanadium, nickel, iron,manganese, germanium, and alloys thereof and the backing plate isselected from the group comprising copper, aluminum, titanium, andalloys thereof and the backing plate is selected from the groupcomprising copper, aluminum, titanium, and alloys thereof.